FreqSLF,
I would surmise that the additional airstream noise and vibration from having landing gear hanging and flaps at full deflection for landing was enough to mask the sound of the engines at approach power. For the jets I've flown, the engine's rotor speed has been in the 60 - 70 % N1 for the approaches, and this is usually much quieter than the up to 100 % one might hear on takeoff or even climb.